The present invention relates to volume measuring sensors, and more particularly to a sensor for measuring the volume of air or ullage in a container or a tank, thereby providing an indication of the volume of a liquid, powder or solid occupying the remaining volume of the container.
Several factors have come to the fore in recent years to suggest that a new design for a fuel quantity gauge is becoming a necessity. With the advent of consumers desiring smaller automobiles, designers would like the flexibility of employing convoluted fuel tanks to achieve space efficiency. This will necessitate a change from the simple fuel level gauges in use today.
Consumers, having voiced their desire for longer and more inclusive warranties, are getting such from automobile manufacturers. As a consequence, manufacturers are looking for methods to lessen repair costs wherever possible. Presently if an automobile is brought in for repair because of a defective fuel gauge, the entire fuel tank is replaced. The cost of dissecting the old fuel tank and repairing the gauge is prohibitive. Manufacturers would like externally mounted or easily removable fuel sensors so that the good fuel tank would not have to be discarded, thereby reducing warranty repair costs.
An unlevel vehicle and/or fuel sloshing contribute sources of error to fuel level gauges. Considering the increasing amount of stops and starts for today's commuting driver, the fuel sloshing could render the fuel gauge inaccurate for a large fraction of the time. This enhances the need for a level and sloshing insensitive fuel quantity sensor.
The new dashboard displays can display a high degree of accuracy in their readouts. So much so, that now the limiting factor in the accuracy of reading the remaining fuel is no longer in the display but in the fuel quantity gauge itself. Car manufacturers would like more accurate fuel gauges.
These four compelling reasons indicate the definite need for a new or improved fuel quantity gauge.
Conventional gauges have been used in the measuring of fuel for years. The automobile with its relatively quiescent journey and limited elevation angle, typically employs the mechanical float sensor. This sensor detects level of fluid in the tank and is inexpensive. A simple mechanical float fuel sensor consists of a float (which always rides at the level of the fuel) and vertical rails which constrain the float. For a reference see E. W. Pike et al., “Investigation of Fuel Quantity Measuring Techniques,” DTIC-AD712120, USAF-AMC Wright Patterson AFB, Ohio, June 1952. This sensor produces either a changing voltage or current as the float moves up and down along the rails. There have been numerous advances in the mechanical float sensor. However, most mechanical float sensors tend to suffer from the following general disadvantages: (1) Mechanical float sensors required some electricity within the fuel tank, disadvantageous from a safety standpoint. (2) Mechanical mechanisms of any sort break down with much higher regularity than any other system having no moving parts. (3) Mechanical floats have lower accuracies than other fuel gauges available today. (4) Mechanical float gauges measure only fuel level. This is a disadvantage as fuel sloshing, inclining of the road, and the convoluted fuel tank shapes of today decrease the accuracy of fuel level as a measure of fuel quantity.
Some aircraft use mechanical float sensors, but most use a coaxial capacitive sensor. Whichever sensor is used, a matrix of these sensors (from 4 to 12, typically) is typically used within each fuel tank inside the aircraft. There are several separate fuel tanks within an aircraft to take best advantage of this limited volume available in the wings and fuselage. This matrix of sensors and averaging electronics is required to allow some measure of accuracy during maneuvering and climbing. The capacitive sensor is also more accurate than the mechanical float sensor and can therefore provide better fuel management and less likelihood of running out of fuel. However the capacitive sensor is more expensive than the mechanical float sensor, making a matrix of such sensors prohibitively expensive for use in automobiles. Microbial growth in the fuel tank has been shown to affect the accuracy of this sensor. For references see W. B. Engle and R. M. Owen, “Electrical and Physical Nature of Microbial Membranes Implicated in Aircraft Fuel Quantity Probe Malfunction,” SAE-710439, National Air Transportation Meeting, Atlanta, Ga., May 1971; J. Huddart, “An Alternative Approach to Fuel gauging,” ASE-790138, Society of Automotive Engineers, Detroit, Mich., February/March 1979; K. Suzuki, T. Tomoda, and S. Momoo, “A Highly Accurate Fuel Level Measuring System,” SAE-871961, Passenger Car Meeting, Dearborn, Mich., October 1987; P. Weitz and D. Sale, “Effects of Anti-Static Additives on Aircraft Capacitance Fuel Gauging Systems, AFWAL Wright Patterson AFB, Ohio, Technical Report #AFWAL-TR-80-2058, June 1980.
A fiber optic liquid level gauge is described in J. W. Berthod, “Fibre Optic Intensity Sensors,” Photonics Spectra, 22(12), 125–138 (December 1988), and utilizes two fibers, a prism, an LED, and a detector. Multiple fiber sensors, each of different length, can be employed to provide an incremental level capability. The disadvantages of the fiber optic fuel gauge are: 1) the sensor must be located inside the tank, 2) films can form on the prism and foul the sensor, 3) the fiber optic sensor is a discrete sensor, and 4) the fiber optic sensor is a level sensor only.
There are two techniques associated with another known fuel sensor, the Boyle's Law or pressure fuel quantity gauge. (For references, see: H. Garner and W. Howell, “Volume Fuel Quantity Gauge” Patent Application, NASA-CASE-Lar-13147-1, Ser. No. 06/643/523 filed Aug. 23, 1984 and now abandoned. Takebayashi, “Volume measurement of liquid in a deformed tank,” SAE-871964, Passenger car meeting, Dearborn, Mich., October, 1987.) The first technique (the Beckman method) uses isothermal compression to measure the volume of the gas. Any isothermal (constant temperature) change in volume is accompanied by a change in pressure. Measuring this pressure change, as a piston which is connected to the system collapses its volume, yields a measure of the entire tank volume. A major drawback of this technique is that it cannot work in a tank that has vent holes or leaks of any kind. Such leaks would not allow the pressure build up that is so critical to the measurement. A second method, proposed by Wantanabe and Takebayashi, id., uses an adiabatic (no heat flow) process and a step function of pressure to determine the volume of the air in the tank. This method can deal with small, medium, and large holes in the tank. The effect of leaks in the tank do not alter the outcome of the gauge; they only modify the relaxation time and damping of the pressure pulse in the tank. By noticing the speed of decay of the pressure after the step response, the gas volume can be determined. The disadvantages of this system are (1) the system is bulky and heavy, (2) the adiabatic system requires more complex electronics, and (3) the pistons and valves involved together with the electronics cause this gauge to be very expensive compared to other automobile fuel gauges.
It is therefore an object of the present invention to provide an improved system for measuring the gas volume in a closed container, which is reliable and relatively inexpensive to manufacture.
A further object is to provide an improved adiabatic pressure system for measuring the quantity of a liquid, solid or powder in a closed container of known empty volume.
Yet another object of this invention is to provide an improved fuel quantity gauge system for vehicles.